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Date: Mon, 30 Oct 2017 17:33:22 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Tobin C. Harding" <me@...in.cc>
Cc: kernel-hardening@...ts.openwall.com, "Jason A. Donenfeld"
 <Jason@...c4.com>, Theodore Ts'o <tytso@....edu>, Linus Torvalds
 <torvalds@...ux-foundation.org>, Kees Cook <keescook@...omium.org>, Paolo
 Bonzini <pbonzini@...hat.com>, Tycho Andersen <tycho@...ker.com>, "Roberts,
 William C" <william.c.roberts@...el.com>, Tejun Heo <tj@...nel.org>, Jordan
 Glover <Golden_Miller83@...tonmail.ch>, Greg KH
 <gregkh@...uxfoundation.org>, Petr Mladek <pmladek@...e.com>, Joe Perches
 <joe@...ches.com>, Ian Campbell <ijc@...lion.org.uk>, Sergey Senozhatsky
 <sergey.senozhatsky@...il.com>, Catalin Marinas <catalin.marinas@....com>,
 Will Deacon <wilal.deacon@....com>, Chris Fries <cfries@...gle.com>, Dave
 Weinstein <olorin@...gle.com>, Daniel Micay <danielmicay@...il.com>, Djalal
 Harouni <tixxdz@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V8 2/2] printk: hash addresses printed with %p

On Thu, 26 Oct 2017 13:58:38 +1100
"Tobin C. Harding" <me@...in.cc> wrote:

> > +static bool have_filled_random_ptr_key;
> > +static siphash_key_t ptr_key __read_mostly;
> > +
> > +static void fill_random_ptr_key(struct random_ready_callback *unused)
> > +{
> > +	get_random_bytes(&ptr_key, sizeof(ptr_key));
> > +	WRITE_ONCE(have_filled_random_ptr_key, true);  
> 
> This usage of WRITE_ONCE was suggested by Jason A. Donenfeld. I read
> include/linux/compiler.h but was not able to grok it. Is this enough to
> stop the compiler re-ordering these two statements? 
> 
> Or do I need to read Documentation/memory-barriers.txt [again]?

No, the WRITE_ONCE does not stop the compiler from reordering those
statements. If you need that, then you need to do:

	get_random_bytes(&ptr_key, sizeof(ptr_key));
	barrier();
	WRITE_ONCE(have_filled_random_ptr_key, true);

and that only works against interrupts. If you need synchronization
across CPUs, then you need smp_mb().

-- Steve

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